In the modern world where peoples of different religions are
in contact with each other it is dangerous to attempt what was
common in the past: eius regio, eius religio
(each kingdom its own religion).

The first explicitly secular state was the United States where
the constitution requires the government to have no views on
religion - not to favor one religion over another. Others are Turkey
where Kemal Ataturk removed the dominant status of Islam. India with a large
Muslim minority is also secular. In both the last states there
are threats from political Islam and Hinduism respectively to set up
a religious state.

The Revolutionaries in France
declared the separation of Church and state.

Before the American Revolution the example the founding fathers
were reacting from was 18th century Britain which had an Established Church,
where the bishops were appointed by the rulers and other religions
forbidden or restricted. In modern Iran and Saudi Arabia Islam is established. The constitution
of the Republic of Ireland mentions the "special status"
of the Roman church - though in practice the Archbishop of Dublin
no longer has the power he once had of intimidating the politicians.

Modern Britain can be regarded as secular. Although the church
is still part of the government ceremonial no-one is compelled
to support it.